I know you want a dust cover and a forward assist but I am very pleased with the S&W M&P15 Sport...which lacks both. I have never missed them. What is there is great. Much better looking parts than my Stag, which I will say worked flawlessly right out of the box. Very high level of fit an finish.
$600. Gives you $200 to play with for accessories before you get to ammo. Might be able to get an optic which gives you an AR with optic for $800.
Just a thought.
Do you have an earlier sport with the better barrel, or the newer one with the lesser (and 1/9) barrel?
Either way, there are a lot of differences between a Sport and some of the other rifles being discussed other than just a dust cover and forward assist